The present invention relates to a pressure converter and more specifically to a pressure converter for detecting soil pressure (here after, referred to as “effective earth pressure”) which eliminates pore water pressure in the ground, such as at the bottom of dams, in river beds, sea beds and sediment slopes, and also for detecting a micro fluctuation of the pore water pressure (hereafter, referred to as “dynamic pore water pressure”) among soil particles forming the ground and then outputting such data as transmission signals.
Naturally, as the ground at the bottom of a dam, river bed, sea bed or sediment slope is repeatedly accumulated and eroded, sediment gravel, dirt and sand determines geological features and influences structures constructed on the ground, which makes it necessary to examine soil pressure (hereafter, referred to as “earth pressure”) in the ground beforehand. The earth pressure generally includes soil skeleton pressure of soil particles forming the ground and pore water pressure among the soil particles. This pressure is generally called “total earth pressure.” In addition, the pore water pressure is the pressure of air and water mixture.
Furthermore, the earth pressure, which excludes pore water pressure from the total earth pressure is called “effective earth pressure.” Therefore, it is possible to determine whether the ground is strong enough for constructing underground structures or whether the slope of the ground is stable by measuring the effective earth pressure or the pore water pressure in the ground. It is also becoming possible to determine whether the surface of sediment dirt and sand has risen or fallen.
A conventional example for conducting such measurement is an electrical pressure converter which the present inventor et al. have proposed in U.S. Pat. No. 2,696,099.
FIG. 12 is a cross-sectional view showing the configuration of an effective earth pressure gauge according to the conventional example (hereafter, referred to as “first conventional example”).
In FIG. 12, at the center of the front and rear surfaces of the strain starting, portion 81, pressure receiving rods 83, 83 are united as one structure or integrally fixed so that the rods coaxially protrude from the main body 82. A sword-guard shaped pressure receiving plate 84, 84 is provided at each top end of the pressure receiving rod 83, 83.
An expandable bellows 85, 85 is mounted between each pressure receiving plate 84, 84 and the main body 82 so that the bellows 85, 85 seals the opening to prevent moisture from sinking into the above-mentioned strain starting portion 81 side. At a central portion and a peripheral edge of both front and rear surfaces of the strain starting portion 81, a plurality of strain gauges 86 are attached to function as elements which convert the amount of deformation of the strain starting portion 81 into an electrical signal, thereby forming the Wheatstone Bridge, not shown in the drawing.
On the outer peripheral edge side of one (the upper one in the drawing) of the above-mentioned pressure receiving plates 84 and bellows 85, a filter 93 is mounted to the main body 82 by a screw-in means or the like so that a hydraulic pressure chamber 88 is defined by the surrounding filter. Furthermore, on the outer peripheral edge side of the other pressure receiving plate 84 and bellows, a portion of the main body 82 is projected like a sword-guard, and a thin pressure-receiving diaphragm 94 is installed on the cross-section of the main body to form a detecting portion 95 which faces the pressure receiving plate 94 with micro pores interposed. The detecting portion 95 is filled with a liquid 96, such as hydraulic oil or the like.
AAn earth pressure gauge, configured as stated above, is designed so that when it is buried in the ground, the surface of the pressure receiving diaphragm 94 receives overalltotal earth pressure including soil skeleton pressure of soil particles and pore water pressure. On the other hand, the filter 93 prevents soil particles from entering but allows only pore water to enter so that the hydraulic pressure chamber 88 receives pore water pressure.
Therefore, between soil skeleton pressure and pore water pressure applied to the pressure receiving diaphragm 94, the pore water pressure is cancelled by the pore water pressure on the filter 93 side, and therefore, only soil skeleton pressure (this is called “effective earth pressure”) is applied to the strain starting portion 81 via the pressure receiving plate 84 and the pressure receiving rod 83. Accordingly, an electrical signal corresponding to the effective earth pressure can be obtained by the strain gauge 86. This earth pressure measurement detects the magnitude of the earth pressure in the ground, which makes it possible to determine the strength of the ground before structures are constructed.
FIG. 13 is a cross-sectional view showing the configuration of a dynamic pore water pressure gauge disclosed in U.S. Pat. No. 2,696,099 (hereafter, referred to as “second conventional example”).
In FIG. 12, on the outer periphery side of the lower pressure receiving plate 84 and bellows 85, a portion of the main body 82 is projected like a sword-guard, and a thin pressure-receiving diaphragm 94 is installed on the cross-section to form a detecting portion 95 which faces the pressure receiving plate 94 with micro pores interposed. However, in FIG. 13, instead of providing the detecting portion 95, a filter 89, having the attenuation characteristic different from the upper filter 90, is mounted to the main body 82 by a screw-in means or the like so that a hydraulic pressure chamber 87 is defined as the result of the filter surrounding the pressure receiving plate 84 and the bellows 85.
As shown in the characteristics diagram in FIG. 14 showing the relationship between the fluctuation frequency (Hz) of the measured pressure and the attenuation factor of the filter [Pb/Po], one filter 89 uses a rough-mesh filter with the A-characteristic curve which does not attenuate pressure waves until a high frequency is reached, and the other filter 90 uses a filter with the B-characteristic curve which attenuates pressure waves at a lower frequency compared to the above-mentioned filter 89. Furthermore, concerning symbols indicating the attenuation factor of the above-mentioned filters, Po denotes an input pressure value and Pb denotes an output signal pressure value.
A dynamic pore water pressure gauge, configured as stated above, is buried in the ground of the seabed, for example. Hydraulic pressure caused by water depth, that is, static hydraulic pressure (hereafter, referred to as “hydrostatic pressure”, is applied through the filters 89, 90 without generating a pressure difference between the hydraulic pressure chambers 87, 88 even if attenuation characteristics of the filters 89, 90 are different; and pare water pressure is uniformly applied to the pressure receiving plates 84, 84. As a result, the strain starting portion 81 is not deformed and the strain gauge 86 does not generate any electrical outputs, and therefore, hydrostatic pressure is cancelled.
However, if dynamic pore water pressure occurs due to the wave having a specific period in the pore water within the ground, the difference in the attenuation characteristics of the filters 89, 90 causes a pressure difference between two hydraulic pressure chambers 87, 88.
That is, as FIG. 14 shows, because dynamic pore water pressure in the hydraulic pressure chamber 87 side which has a filter 89 having the A-characteristic curve becomes high, the pressure deforms the strain starting portion 81 via a pressure receiving plate 84, and the amount of deformation of the strain starting portion 81 is detected as an electrical resistance value by a strain gauge 86. Accordingly, it is possible to detect an electrical signal corresponding to the dynamic pore water pressure from the output end of the bridge formed by the strain gauge.
As a consequence, regardless of the magnitude of the hydraulic pressure caused by water depth or earth pressure, it is possible to measure a micro fluctuation of the dynamic pore water pressure caused by an earthquake or other movements of the earth's crust. Furthermore, the dynamic pore water pressure data indicates the strength of the ground, which is an excellent indication used for the design, construction and safety management of the structures to be constructed.
The effective earth pressure gauge according to the above first conventional example and the dynamic pore water pressure gauge according to the above second conventional example enable the measurement of the effective earth pressure and the dynamic pore water pressure. However, each of those conventional examples uses two bellows 85, 85 having the same spring constant and applies the configuration of a differential pressure gauge in which the strain starting portion 81 located inside those bellows 85, 85 is deformed due to a liquid pressure generated outside the bellows 85.
In the configuration that uses two bellows 85, 85, as FIGS. 12 and 13 show, there is a head difference between the height from the differential pressure center position to the bellows' pressure load position A and the height from the differential pressure center position to the bellows' pressure load position B, which affects the measured pressure. Therefore, this becomes a problem when generated effective stress has to be measured highly accurately with an error of less than 1-CM head.
If the device shown in FIGS. 12 and 13 is rotated 90-degrees and installed, the head difference can be eliminated. In that case, however, when effective earth pressure is measured, symmetry in the vertical earth pressure direction is deformed, and when dynamic pore water pressure is a measured, the transverse rectangle shape makes it difficult to insert and install the device into the ground, that is, generally through a small-diameter bore hole.
Moreover, in fact, since it is difficult to obtain two bellows which have quite the same spring constant, bellows which have approximately similar spring constant may be used. Due to the difference between those spring constants, an interference output which has nothing to do with the hydrostatic pressure caused by the depth of installation is applied to the bellows as an interference output. In an effective earth pressure gauge, as FIG. 12 shows, earth pressure is applied from the pressure receiving diaphragm 94 to one bellows 85 via the pressure of the liquid 96 that fills in the diaphragm chamber; and in some cases, the effect cannot be ignored.
Recently, a plurality of effective earth pressure gauges and dynamic pore water pressure gauges are coupled to measure the effective earth pressure and the dynamic pore water pressure distributed in the depth direction.
However, if a plurality of bellows-type pressure converters according to the above-mentioned first and second conventional examples is are coupled in the multi-stage arrangement, the problems described below arise.
That is, if a first pressure converter 110, a second pressure converter 120, a third pressure converter 130 and so on are sequentially connected in the multi-stage arrangement, as shown in FIG. 15, via connecting pipes 114, 124, 134, signal cables 111, 121, 131 must be extended to the other sides along the outer surface of the pressure converters 110, 120, 130.
In addition, to protect each signal cable 111, 121, 131, a protective member 113, 123, 133, and a protective pipe 112, 122, 132 that is connected to the protective member must be installed so that the signal cable 11, 121, 131 can pass through the protective member and the protective pipe.
As FIG. 15 shows, the signal cable 111, 121, 131 extends from the connecting pipe 114, 124, 134, passes through the protective members 113, 123, 133, and then extends along the outer circumferential surfaces of the cylindrical main bodies of the first through third pressure converters 110 through 130 and the filters 115, 125, 135 while being covered by the protective pipe 112, 122, 132. Accordingly, the configuration around the cable becomes large, which may disturb the observed soil pressure or the signal cable may be exposed to an environment where it is easily abraded.